Digitizing tablets are well known in the art. They employ a variety of technologies. See, for example, the description in the January, 1989 issue of BYTE magazine, pages 162-174. The typical tablet comprises a regular array of electrode wires mounted in the tablet underneath its surface and extending generally parallel to an X, Y coordinate grid. In a typical construction, the electrodes are energized and emit a signal which is picked up by a pointing device positioned on or over the tablet surface. Processing of the signals from the cooperating electrodes and pointing device returns a pair of digitizer values representative of the X, Y coordinates of the pointing device location. Pointing devices typically come in the form of a cursor or a stylus, and for convenience the term stylus will be used to generally refer to any kind of pointing device that can cooperate with the electrode array to generate the desired coordinate signals. One of the significant parameters involved in this mechanism is what is known as stylus proximity. Since the interaction between the electrodes and stylus is by way of an electromagnetic or electrostatic field, stylus proximity defines how far the stylus can be raised above the tablet before it stops generating coordinate data. The tradeoff here is between accuracy and proximity. The larger the proximity value, the more accuracy or resolution suffers, because of the spreading of the fields. Hence, it is conventional for the tablet manufacturer to determine at the factory the proximity threshold level above which the tablet will no longer process data, as it would fall outside its resolution specification. However, these digitizer tablets are frequently used for the purpose of digitizing drawings, in which application the user places the drawing on the top of the tablet surface and traces the lines with the stylus. Thick drawings naturally place the stylus further away from the tablet surface. If the proximity threshold level has been set by the manufacturer at, say, 1/4 inch, then the user will not be able to trace a drawing that is thicker than 1/4 inch.
Another problem associated with the way in which the user operates the stylus is the control over the stylus functions. For instance, it is common to include button activated electrical switches and the like so that the user can "turn on" the stylus and "turn off" the stylus at will. This is to prevent the digitizer recording stylus positions in which the stylus is approaching or withdrawing from the drawing to be traced, which the user can control by only activating the tablet when the stylus is actually in contact with the drawing. As an alternative to these more expensive switch-activated systems, the tablet can be adjusted at the factory with a very small proximity threshold level. But again the versatility of the device is sacrificed because then only drawings on thin paper can be copied.
Another problem arises in connection with stylus pressure exerted by the user on the tablet surface. Some users would prefer, for example, to rest their hand on the tablet while pressing the stylus in order to more accurately follow the contour of the drawing. Each user has his own style. Yet, the versatility of the tablet is restricted as a result of the tablet manufacturer setting at the factory the pressure threshold level above which the tablet is activated. As a result, the users have to adjust their style to the tablet settings.
It is evident from the forgoing description that a need exists in the art for a digitizer tablet in which the user can, so-to-speak, customize the stylus proximity and the stylus pressure parameters to his own style or to the application. The obvious solution to this problem is to place manual controls on the tablet that would allow the user to manually set the proximity and pressure threshold levels. This, however, is an expensive solution requiring more parts and more circuitry and also an undesirable tablet appearance.
Another solution to the problem is described in commonly assigned U.S. Pat. No. 4,788,386, of which I am one of the joint inventors. The tablet described in that patent, whose contents are hereby incorporated by reference, employs electrostatic fields for determining stylus location. In particular, the stylus is capacitively coupled to the electrodes. The proposal in that patent is to provide along the outside of the tablet separate from the regular electrode array, a series of additional electrodes in different geometric patterns, termed menu electrodes. These menu electrodes are connected to the regular position determining electrodes. The signals from these special menu electrodes can be recognized and distinguished from the position coordinate signals by signal magnitude. The menu electrodes are given a different geometry from the position determining electrodes and therefore would generate a signal of different magnitude. When it is determined that a menu signal has been generated then, as this patent explains, a specific software routine can be invoked to modify the processing of the signals. Among the modified processing possible described is varying the stylus proximity threshold and pressure threshold levels. Other parameters could also be varied. A disadvantage of this approach is the requirement for including these special geometric menu electrodes into the tablet.